The present invention relates to a new process for preparing dithiine-tetracarboxy-diimides.
Dithiine-tetracarboxy-diimides as such are already known. It is also known that these dithiine-tetracarboxy-diimides can be used as anthelmintics against internal parasites of animals, more particularly nematodes, and have insecticidal activity (cf. U.S. Pat. No. 3,364,229). It is known, furthermore, that certain dithiine-tetracarboxy-diimides possess antibacterial activity and have a certain activity against human mycoses (cf. Il Farmaco 2005, 60, 944-947). It is also known that dithiine-tetracarboxy-diimides can be used as fungicides against phytopathogenic fungi in crop protection (cf. WO 2010/043319). It is known, furthermore, that dithiine-tetracarboxy-diimides can be used as pigments in electrophotographic photoreceptors or as dyes in paints and polymers (cf. JP-A 10-251265, PL-B 143804).
Dithiine-tetracarboximides of the formula (I)
    in which    R1 and R2 are identical or different and are hydrogen, or are C1-C8-alkyl which is optionally substituted one or more times by halogen, —OR3, and/or —COR4, are C3-C7-cycloalkyl which is optionally substituted one or more times by halogen, C1-C4-alkyl or C1-C4-haloalkyl, or are aryl or aryl-(C1-C4-alkyl) each of which is optionally substituted one or more times by halogen, C1-C4-alkyl, C1-C4-haloalkyl, —COR4 or sulphonylamino,    R3 is hydrogen, C1-C4-alkyl or C1-C4-alkylcarbonyl or is aryl which is optionally substituted one or more times by halogen, C1-C4-alkyl or C1-C4-haloalkyl,    R4 is hydroxyl, C1-C4-alkyl or C1-C4-alkoxy,    can be prepared in a variety of known ways.
For example, in one known process (cf. Synthetic Communications 2004, 36, 3591-3597), in a first stage, succinic anhydride is reacted with an amine of the formula (II), optionally in the presence of a diluent. Subsequently, the resultant succinic monoamides of the formula (III) are then reacted with a large excess of thionyl chloride in the presence of dioxane as diluent at room temperature, to give, finally, in a sequence of numerous reaction steps, the dithiine-tetracarboxy-diimides of the formula (I). The dithiine-tetracarboxy-diimides are optionally isolated directly from the reaction mixture or by filtration following addition of water. Depending on reaction conditions (diluents) and the nature of the radicals R, it is possible in certain circumstances to isolate the dithiine-diisoimides of the formula (IV) before they are converted into the dithiine-tetracarboxy-diimides of the formula (I):

Disadvantages of this process are the long reaction time and also the outcome where either the yields obtained generally do not exceed about 30-40% of theory or else the purities of the isolated products are inadequate. A further disadvantage, in the case of aqueous work-up of the reaction mixture, is that it involves destroying large amounts of thionyl chloride; the gases formed (SO2 and HCl) have to be disposed of. Likewise a disadvantage is the fact that, from experience, the product is not obtained in one portion. Instead, it is frequently the case that, following initial isolation of product by filtration, further product precipitates from the filtrate after prolonged standing (overnight, for example), and must be isolated again by filtration. Occasionally this operation must be carried out once more. This procedure is very laborious and time-consuming.
In another known process (cf. U.S. Pat. No. 3,364,229; Chem. Ber. 1967, 100, 1559-70), in a first stage, dichloromaleic anhydride of the formula (V) is reacted with an amine of formula (II), optionally in the presence of a diluent. Subsequently, the resultant dichloromaleimides of the formula (VI) are then reacted with a sulphur donor compound (for example hydrogen sulphide, thiourea or sodium thiosulphate):

This process has the disadvantages that, for example, operating with the highly toxic gaseous hydrogen sulphide is from a technical standpoint very difficult, costly and inconvenient. When thiourea is used, unwanted by-products are obtained along with the target product, and are very difficult to remove and detract from the attainable yields. If sodium thiosulphate is used, the yield described is insufficient for an industrial operation. Similar considerations apply in respect of the embodiment of the reaction that is described in U.S. Pat. No. 3,364,229, with sodium sulphide (see Example Xb therein).
The preparation of certain dithiine-tetracarboxy-diimides of the formula (I) by reaction of the corresponding dichloromaleimide of the formula (VI) with sodium sulphide is also known, furthermore, from Revue Roumaine de Chimie 2005, 50, 601-607. Here as well, however, the yields are inadequate.
Consequently there continues to be a need for a technically simple and economic preparation process for dithiine-tetracarboxy-diimides of the formula (I).